6. Patterned LEDs
6.2 White LED characterization
Photoluminescence (PL) measurement obtained with a 405 nm pump laser is presented in Figure 6.6. This sample has InP/ZnS NCs deposited inside holes in the patterned area. Good color mixing results for the patterned devices are shown. The color rendering index (CRI) has a reasonable value of 73 and CCT of 6190 K (values were calculated using the SpecWin software from “Instrument System”
company, supplied with the spectrometer), which supports the assumption that good quality white light can be obtained from hybrid white LEDs where NCs will substitute phosphor. Note that higher CRI index would be obtained if the NCs of different sizes, thus different emission wavelengths, were used to extend the emission beyond 600 nm wavelength. It should be mentioned, that the data was taken with a 420 nm long-pass filter needed to remove the signal from the excitation laser beam resulting in an abrupt cut-off of the luminescence for shorter wavelengths.
400 500 600 700
0,00 0,25
0,50 Absorption and emission spectra of NCs
Wavelength (nm)
136
Figure 6.6. Spectrum of an optically pumped white LED.
As the laser beam size used in this experiment was rather large (around 100 µm in diameter) it was not possible to establish the role of RET in the total emission. In order to see the effect of patterning and since the interactions between the NCs and the QWs take place on the etched surfaces of the openings, we performed spatially resolved photoluminescence. Spatially- and time-resolved PL (TRPL) measurements discussed further in section 6.3 are particularly suitable for establishing the presence of RET between the QW and the NCs.
EL measurements performed on this sample didn't show red emission from the NCs. We attribute it partly to low absorption coefficient for the InP/ZnS NCs at the emission wavelength of the LED and partly to the charge trapping and subsequent Auger recombination that were limiting the performance of NCs [116]. Optically driven diodes don’t suffer from these problems that much. NCs have higher absorption coefficient for emission wavelength of the laser. Moreover, intensity of the laser emission is high enough to overcome charge trapping and other non-radiative processes in the NCs and supply
400 500 600 700
137
enough carriers for the radiative recombination. More importantly, if the device is excited with a pulsed laser with the period longer than the decay rate of carriers in both QW and NCs, the ratio of emission between the two is defined by the ratio of the absorption probabilities and the quantum yield of QW and NCs. Under electrical excitation (which is CW) the number of photons emitted by the QW is defined by the current, while number of photons emitted by the NC is defined by the absorption of QW emission by NCs at low excitation levels and by the decay time of the carriers in QW and NC at the intermediate excitation level. At low excitation level the number of photons emitted by the NCs will increase linearly with the current through the LED and the intensity ratio should reflect the absorption probability. At the intermediate excitation level, when there is a high chance of photon absorption during the exciton lifetime, Auger recombination will start limiting the number of photons emitted by the NCs. Finally, at very high excitation levels, the number of red photons emitted by the NCs will be defined by the number of the NCs and the lifetime of the exciton (~20 ns) and will be current independent. The decay time of the QW is of the order of 4 ns and decay time of NCs is of the order of 13 ns. Auger recombination tends to be very fast, occurring with the decay time of sub-ns.Following this observation devices with CdSe/ZnS NCs as energy acceptors were prepared in order to substitute InP/ZnS NCs with more efficient ones.
A typical current-voltage characteristic of the LED is presented in Figure 6.7, where the inset shows the low voltage part of the curve on a µA scale. The device has a good turn-on voltage value of around 4.2 V, but starts emitting light intense enough to see by naked eye at 2.6 V.
Patterning followed by post-etching treatment didn’t affect the IV
138
characteristics. This confirms that surface damage caused by ICP etching was successfully removed and patterning using etching techniques may be used without sacrificing the electrical characteristics of the device.Figure 6.7. IV characteristic of the device. Inset shows low current and low voltage part of the curve.
It should be noted, that the amount of the solution with CdSe NCs deposited on a sample was the same as amount of the solution with InP NCs. It resulted in higher emission from the CdSe NCs. Figure 6.8 presents PL and EL measurements of the patterned device with CdSe/ZnS NCs covering conformally the surface of the LED. For the optical measurements the same blue laser emitting at 405 nm together with 405 nm notch filter were used. Optically driven sample exhibits strong emission from the NCs, but driven electrically it shows negligible color conversion of blue to the red emission via optical absorption and RET. Since the geometry of the device allows only small number of NCs to interact with the QW carriers, it is not expected that RET would be significant.
139
400 500 600 700
0 6000 12000
400 500 600 700
60 90 120
Counts (a.u.)
Wavelength (nm) w19.4-p8.1-348-722nm-EL
Intensity (a.u.)
Wavelength (nm)
EL PL
Figure 6.8. PL and EL measurements of the patterned device with CdSe/ZnS NCs in close vicinity to the QW.
Measurements of electrically driven diode showed higher overall emission from the patterned area. This can be seen even better in Figure 6.9, where the patterned areas are the brightest rectangular emission spots in the picture. Holes themselves help to extract light from the QWs, thereby increasing the external efficiency of blue emission [78], [146].
140
Figure 6.9. A photo of electrically driven device, two probes provide current to the electrical contacts. As generated photons are guided in GaN, light is emitted from large part of the chip around the contact device. Note, that the arrays of hole patterns are always brighter than the surrounding unpatterned parts of the
same mesas.
Global characteristics of electrically and optically driven diodes have shown complicated behavior of devices. Time-resolved PL analysis performed with high spatial resolution has to be done. This analysis will give information about carrier dynamics in the small region where NCs are in close vicinity to the QWs instead of gathering signal from the area of tens of µm.